The pathogenesis of splenic extramedullary hematopoiesis in metastatic carcinoma

Splenic extramedullary hematopoiesis (EMH) is a characteristic finding in agnogenic myeloid metaplasia (AMM) and in the spent phase of polycythemia vera (PV). Evidence from our laboratory has suggested that splenic EMH in these conditions results from the filtration of circulating hematopoietic cells from the peripheral blood and does not arise de novo from splenic stem cells. To further test this hypothesis, 31 autopsy and 26 surgical cases of carcinoma metastatic to the bone marrow were studied. The presence of leukoerythroblastosis (LEB) correlated with intravascular hematopoiesis (IVH) in the bone marrows associated with reticulin fibrosis, and with splenic EMH in the autopsy cases. These studies provide evidence that stromal changes in the bone marrow with resulting IVH, LEB, and splenic EMH are not unique to AMM and PV but also occur in such unrelated conditions as metastatic carcinoma, and suggest that these phenomena are causally related.